Exported model from Blender has holes when imported in Unity
Unity does Backface culling by default whereas Blender only does it on request. The issue is that the normals are flipped. If you want to turn on Backface culling in Blender it is in the Properties panel.

Here I have removed a face from the default cube.


and now with Backface culling enabled.


You can see through the faces that are facing outwards. To fix this, i.e. making a room, you can flip the normals so the faces face inwards.